In some optical applications, there is a need to change the phase difference between two waveguides, such as waveguides on planar substrates or the like (e.g. optical fibers) as fast as possible. The phase difference can be tuned by changing the optical length (length times the refractive index) in either one or both waveguides so that the optical length difference between them changes.
For example, in an optical 2×2 switch based on a Mach-Zehnder interferometer (MZI), as seen in FIG. 1, a phase difference change between two adjacent waveguides 3 and 4 in the area between directional couplers 16 and 17, induced by heating, makes the switch shift back and forth between the bar and cross states. In the bar state (on-state), the optical power coming from one input is directed to the output of the same waveguide, while in the cross state (off-state) the same power is directed to the output of the adjacent waveguide. If both directional couplers 16 and 17 of the inputs and outputs of the switch are ideal 50:50 power splitters, then the switch is in a cross state (off-state) when the phase difference is 0° (±N·360°) and in a bar state (on-state), when the phase difference is 180° (±N·360°). Between the bar and cross states, the coupling state of the switch changes as a cosine function of the phase difference. If directional couplers are non-ideal 50:50 power splitters, then at least one of the bar and cross states is only partial, in which case such a partial coupling state the output power of neither waveguide is zero and the optical power is split to both outputs in a certain proportion. When the directional couplers 16 and 17 are lossless and mutually identical, the transmission Tx of a waveguide that is crosswise with respect to the input is as a function of the phase difference Δφ according to
            T      x        =                  1        2            ⁢                                    sin            2                    ⁡                      (                          π              ⁢                                                          ⁢                              r                /                2                                      )                          ⁡                  [                      1            +                          cos              ⁡                              (                                  Δ                  ⁢                                                                          ⁢                  ϕ                                )                                              ]                      ,
where r is the length of the directional coupler in relation to its ideal length. The transmission of the bar state is T||=1−Tx. By using only one input or output port of the aforementioned switch, or by using a symmetrical 1×1 Mach-Zehnder interferometer operating in the same manner, one can realise e.g. a tunable attenuator or an on/off switch. With similar structures one can also realise e.g. tunable wavelength filters.
FIG. 2 represents a schematic cross-section of a known switch, which was already illustrated in FIG. 1. In this example, waveguides 3 and 4 are so called silicon-on-insulator (SOI) waveguides. The switch includes a substrate 12 made of silicon (Si) and which is, in this example, approx. 0.5 mm thick. On top of the substrate 12 lies a thin SiO2 layer 13, which is 1 μm thick. On top of the SiO2 layer 13 is an approx. 5 μm thick silicon slab (Si) 14 covering the whole surrounding of the waveguides. The waveguides 3, 4 are defined by local ridges. Along the ridges the thickness of the silicon layer 14 is 10 μm. On top of the silicon layer 14 is a 1 μm thick SiO2 layer 15. Along the waveguides in positions illustrated in FIG. 1, there are 0.5 μm thick heating resistors 5 and 6 on top of the SiO2 layer 15. The ridge acts as a waveguide and the field, illustrated by the dashed line, propagates along the ridge. Horizontally the waveguide is only bound by the steps, so that the silicon slab 14 extends all the way to the other waveguide. The light remains in the position of the ridge and propagates along the ridge.
For controlling an optoelectronic component, two different methods are previously known. These methods are schematically illustrated in FIGS. 3 and 4.
FIG. 3 represents a control signal amplitude of the switch as a function of time, when only one of the two adjacent waveguides is modulated with this first control signal, which is electric and substantially rectangular, and which produces a change in the refractive index (that is, in optical length change) and, thus, produces a phase difference between the waveguides. The amplitude of the first control signal is represented in FIG. 3 by a thick black line. The phase difference induced between the waveguides is represented by a dotted line. In the off-state of the switch the amplitude of the first control signal is zero and in the on-state it is in such a value that the optical length of the modulated waveguide has become shorter or longer by half a wavelength (phase difference 180°).
FIG. 4 illustrates another previously known method, which is an alternative to the method illustrated in FIG. 3, where one of the waveguides is being modulated with an electric control signal that substantially consists of two rectangular parts. In this method, the phase difference between the two waveguides can be raised from zero to the desired target value faster than in the previously described first known method. The higher and substantially rectangular first part of the control signal induces a very fast temperature rise in one of the waveguides, because its peak amplitude is significantly higher than what is needed to maintain the waveguide in its target temperature. Compared to the first known method, this method consumes more power, but it has the advantage of faster rise time.
The methods represented by FIGS. 3 and 4 represent the technology which is closest to the invention and correspond to the preambles of claims 1 and 6. According to the known methods, the refractive index of the first waveguide is changed periodically with the first control signal, the amplitude of which is changed periodically between a first amplitude level I, which is substantially zero, and a second amplitude level II, which is higher than the first amplitude level. In the beginning of the rise time period the amplitude of the first control signal can go to a fourth amplitude level (IV), which is distinctly higher than the second amplitude level. When the first control signal is on the first amplitude level I, which is substantially zero, the refractive indices of the first and second waveguide are equal and the phase difference between them is zero. When the first control signal is on the second amplitude level II, the refractive indices of the first and second waveguide are unequal so that the phase difference is in a predetermined target value. In the rise time period of the phase difference the first amplitude level I forms a start level for the first control signal and the second amplitude level II forms its target level. Similarly, in the fall time period of the phase difference the second amplitude level II forms a start level for the first control signal and the first amplitude level I forms its target level.
Furthermore, it is known that the optical length of a waveguide (and the phase of the light propagating along the waveguide) can be changed e.g. by heating, stressing or bending the waveguide, by producing an electric field into the waveguide or by injecting current through the waveguide. Different modulation mechanisms have their advantages and disadvantages with respect to e.g. speed, optical attenuation, electric power consumption, necessary modulation length and costs.
Known thermo-optical switches usually operate with frequencies reaching up to 1 kHz, at the most, but they are relatively simple and inexpensive to fabricate. Their modulation speed is limited by the heat conduction from the heating resistor to the waveguide core and onwards away from the core, as well as by the heat capacity of the waveguide. In general, heating is more efficient and faster when the volume to be heated is smaller. Good thermal conductivity away from the waveguide, e.g. to an underlying cooled substrate, makes the modulation faster, but it also increases the electric power consumption. If a waveguide is small and efficiently thermally insulated from the surrounding, it can heat up fast but cool down slowly. In general, thermo-optical switches heat up significantly faster than they cool down. However, a silicon-on-insulator (SOI) waveguide represented in FIG. 2, for example, heats up and cools down almost as fast, because the heat efficiently spreads along the horizontal silicon and then conducts from a broad area through the thin oxide layer into the silicon substrate. Experiments have shown that due to the good thermal conductivity in SOI-waveguides, the back and forth 180° phase difference changes can be obtained with frequencies reaching up to 10 kHz, which is somewhat faster than in the commercial thermo-optic switches. While modulating one waveguide, the heating power is then approx. 0.3-0.4 W in the on-state and 0 W in the off-state, which is still quite reasonable. The temperature of the waveguide stabilises exponentially, so that, for example, a 90% modulation can be obtained much faster than a 99% modulation.
Furthermore, it is known that a control signal can be used to create an electric field into the waveguide or to produce an electrical current through the waveguide, which enables the realisation of significantly faster switches, but also these have some typical disadvantages, such as higher optical attenuation and higher costs of the technology. These methods also have a finite delay that limits the modulation speed.
Publication U.S. Pat. No. 5,173,956 describes an optical switch in which the refractive index is controlled by injecting electric current through the material for obtaining internal heating. The publication mentions that the associated switch can reach 1 MHz switching speed. As mentioned above, current injection has the disadvantage of increased optical attenuation.
Publication U.S. Pat. No. 6,278,822 involves an apparatus where there are different materials between two waveguides and a current injected through the materials simultaneously heats up one waveguide and cools down the other waveguide by exploiting the Peltier effect. When applied to an optical switch, this solution can reach 10 MHz switching speed. The disadvantage of this method is that it requires significant changes in the switching structure and cannot, therefore, improve the switching speeds of existing switches.
Publication U.S. Pat. No. 6,351,578 describes an optical switch where the refractive index is changed by heating it with a first control signal that is illustrated in FIG. 4. The operation of the associated switch is not particularly sensitive to the exact values of the control signal amplitudes, because the refractive index change of the switch only needs to exceed a given threshold value for deflecting light out of the waveguide. The method only reduces the rise time of the switch and it has not been applied for changing the phase difference between two adjacent waveguides or for accurate tuning of the refractive index.